In the integrated circuit (IC) industry, it is advantageous to form extremely thin gate oxide layers. One method used by the industry to form thin gate oxides is to decrease oxidation time and/or decrease the gate oxide processing temperature. Any time oxidation time or temperature is reduced, the electrical properties of the gate oxide are effected and controllability of the process is compromised. Therefore, an alternative solution to altering temperature or time of exposure is desired.
In addition, it is advantageous in the integrated circuit industry to form electrical devices that have gate oxides with differing thickness. Prior art methods for forming two devices which have gate oxides with different thickness are extremely complex, thereby resulting in increased process-induced damage and defects.
The prior art has attempted to grow thin oxides by introducing nitrogen into a surface of a substrate. Nitrogen incorporated into a substrate via thermal diffusion or ion implantation will result in a retardation of oxide growth whereby a thinner oxide is formed. However, implantation and thermal diffusion of nitrogen into a substrate surface requires additional processing. The incorporation of nitrogen is not insitu and may result in non-uniform threshold voltage properties of MOS transistors if the nitrogen is not uniformly distributed across the active area of the substrate or if nitrogen is incorporated into the substrate in too large of a concentration. Therefore, the exsitu incorporation of the nitrogen into a substrate in order to retard oxide growth is not, by itself, optimal.
The integrated circuit industry has formed ICs containing two types of transistors with different oxide thickness in order to result in current gain differential devices as taught in U.S. Pat. No. 5,371,026. In these prior art techniques, extremely complex processing technology is utilized to result in two transistors having different electrical characteristics due to different gate oxide properties/thickness. The use of these complex processes is not advantageous due to an impact upon integrated circuit (IC) yield. Therefore, an improved process for forming a single gate oxide having different thickness is desired in the integrated circuit art where one of the oxide thickness is very thin and well-controlled.
In addition, the integrated circuit (IC) industry has studied carbon-based plasma effects on contact resistance and other substrate properties. This study is reported in a paper by Hashimi and titled "The Study on the Influence of Gas Chemistry and Ion Energy for Contact Resistance". In this paper, Hashimi states that carbon incorporation into a substrate due to carbon-based plasma etching is disadvantageous from a contact resistance perspective. Therefore, the integrated circuit (IC) industry, through Hashimi, has suggested that carbon-based oxide-etching plasmas be replaced with non-carbon-based plasmas to avoid increased contact resistance to the substrate. Therefore, the integrated circuit (IC) industry is teaching away from the use of carbon incorporation into a substrate through the use of carbon plasma etch chemistries.
A need exists for a method for forming a single gate oxide layer which has both a thin oxide portion and a thicker oxide portion whereby yield is not impacted, threshold voltages are stable, and integrated circuit processing is not complicated.